It is standard practice to treat diabetes mellitus predominantly with insulin injections to compensate for the inability of the pancreas to make insulin to regulate blood glucose levels. The more tightly a person with diabetes is able to regulate his or her blood sugar, the less detrimental the disease is to overall health. The regulation of blood glucose would benefit from a glucose sensing device implanted in the body to monitor blood glucose levels at more frequent intervals than can be done with presently available repeated blood sampling.
A variety of biomedical measuring devices are routinely used by physicians and clinicians to monitor physiological variables such as respiratory rate, blood pressure and temperature. In addition to the repertoire of devices listed above is the enzyme electrode. Enzyme electrodes enable the user to determine the concentration of certain biochemicals rapidly and with considerable accuracy by catalyzing the reaction of a biochemical and a detectable coreactant or producing a product that may be readily sensed by well-known electrodes (e.g. oxygen, H2O2). Currently there are enzyme electrodes that can detect urea, uric acid, glucose, various alcohols, and a number of amino acids when used in certain well-defined situations.
A number of variations of the glucose enzyme electrode have been developed, all based on the same reaction catalyzed by glucose oxidase.
To accurately measure the amount of glucose present, both oxygen and water must be present in excess. As glucose and oxygen diffuse into an immobilized membrane phase, the glucose reacts with oxygen and water to produce H2O2 (hydrogen peroxide). Glucose is detected electrochemically using the immobilized enzyme glucose oxidase coupled to an oxygen- or hydrogen peroxide-sensitive electrode. The reaction results in a reduction in oxygen and the production of hydrogen peroxide proportional to the concentration of glucose in the sample medium.
The electrode can be polarized cathodically to detect residual oxygen not consumed by the enzymatic process, or polarized anodically to detect the product of the enzyme reaction, hydrogen peroxide. A functional device is composed of at least two detecting electrodes, or at least one detecting electrode and a reference signal source, to sense the concentration of oxygen or hydrogen peroxide in the presence and absence of enzyme reaction. Additionally, the complete device contains an electronic control means for determining the difference in the concentration of the substances of interest. From this difference, the concentration of glucose can be determined.
The enzyme catalase may be included in the oxygen-based system in excess in the immobilized-enzyme phase containing the glucose oxidase to catalyze the following reaction:
Hence, the overall reaction becomes:
This mixture of immobilized enzymes can be used in the oxygen-based device, but not the peroxide-based device. Catalase prevents the accumulation of hydrogen peroxide which can promote the generation of oxygen free radicals that are detrimental to health.
Glucose measuring devices for testing of glucose levels in vitro based on this reaction have been described previously (e.g. Hicks et al., U.S. Pat. No. 3,542,662) and work satisfactorily as neither oxygen nor water are severely limiting to the reaction when employed in vitro. Additionally, a number of patents have described implantable glucose measuring devices. However, certain such devices for implantation have been limited in their effectiveness due to the relative deficit of oxygen compared to glucose in tissues or the blood stream (1: 50-1000).
Previous devices (e.g. Fisher and Abel) have been designed such that the surface of the device is predominantly permeable to oxygen, but not glucose, and is in contact with the enzyme layer. Glucose reaches the enzyme layer through a minute hole in the oxygen-permeable outer layer that is in alignment with an electrode sensor beneath it. Hydrogen peroxide produced by the enzyme reaction must diffuse directly to the sensing anode or through a porous membrane adjacent to the electrode, but is otherwise substantially confined within the enzyme layer by the oxygen-permeable layer resulting in unavoidable peroxide-mediated enzyme inactivation and reduced sensor lifetime.
The strategy of designing devices with differentially permeable surface areas to limit the amount of glucose entering the device, while maximizing the availability of oxygen to the reaction site, is now common (Gough, U.S. Pat. No. 4,484,987). An example based on device geometry is seen in Gough, U.S. Pat. No. 4,671,288, which describes a cylindrical device permeable to glucose only at the end, and with both the curved surface and end permeable to oxygen. Such a device is placed in an artery or vein to measure blood glucose. In vascular applications, the advantage is direct access to blood glucose, leading to a relatively rapid response. The major disadvantage of vascular implantation is the possibility of eliciting blood clots or vascular wall damage. This device is not ideal for implantation in tissues.
An alternative geometrically restricted device assembly was described in Gough, U.S. Pat. No. 4,650,547. The patent teaches a “stratified” structure in which the electrode was first overlaid with an enzyme-containing layer, and second with a non-glucose-permeable membrane. The resulting device is permeable to oxygen over the entire surface of the membrane. However, glucose may only reach the enzyme through the “edge” of the device in a direction perpendicular to the electrode, thus regulating the ratio of the access of the two reactants to the enzyme.
Devices have been developed for implantation in tissue to overcome potential problems of safely inserting into, and operating sensors within, the circulatory system (e.g. Gough, U.S. Pat. No. 4,671,288); however, their accuracy may be limited by the lower availability of oxygen in tissues. The device membrane is a combination of glucose-permeable area and oxygen-permeable domains. The ratio of the oxygen-permeable areas to the glucose-permeable areas is somewhat limited due to the design.
To avoid geometric restrictions on devices, membranes that are variably permeable to oxygen and glucose have been developed (Allen, U.S. Pat. No. 5,322,063). Membrane compositions are taught in which the relative permeability of oxygen and glucose are manipulated by altering the water content of a polymeric formulation. The disadvantages of such a membrane may include sensitivity of the membrane performance to variables during manufacture and that regions of oxygen permeability may not be focused over electrodes within the device.
An alternative strategy to device construction is to incorporate an enzyme-containing membrane that is hydrophilic and also contains small hydrophobic domains to increase gas solubility, giving rise to differential permeability of the polar and gaseous reactants (e.g. Gough, U.S. Pat. Nos. 4,484,987 and 4,890,620). Such membranes readily allow for the diffusion of small apolar molecules, such as oxygen, while limiting the diffusion of larger polar molecules, such as glucose. The disadvantage is that the amount of hydrophobic polymer phase must be relatively large to allow for adequate oxygen permeability, thereby reducing the hydrophilic volume available for enzyme inclusion sufficient to counter inactivation during long-term operation.
Schulman et al. (U.S. Pat. No. 5,660,163) teach a device with a silicone rubber membrane containing at least one “pocket” filled with glucose oxidase in a gelatinous conductive solution located over a first working electrode. In a preferred embodiment, the length of the “pocket” is approximately 3 times its thickness to optimize the linearity between current and the glucose concentration measurement. Because the long axis of the “pocket” is oriented parallel to the electrode surface, this design may be less amenable to miniaturization for tissue implantation.